If you thought airlines could find no new ways to squeeze more passengers into each plane, you are underestimating the resolve of the airline industry.

At this month's Aircraft Interiors Expo in Hamburg, Germany, many of the 500 exhibitors were promoting new ideas to cut down on weight - to save fuel - and innovative layouts to fit more seats per cabin.

Among the concepts offered at the expo was a set of seats that put passengers face to face; seats that are installed in a staggered, diagonal layout, and bathrooms designed to wedge in a few extra passengers in the back of the cabin. One company has even introduced a lightweight lap belt.

Airlines may eventually pack in so many seats per cabin that carriers will reach the maximum passenger totals allowed by federal regulators. But US carriers have not reached that point yet.

"There is no question that densification - adding more seats to each aircraft - is an ongoing trend, and there is no sign of it letting up any time soon," said Seth Kaplan, managing partner at the trade publication Airline Weekly.

One of the world's largest airline interior manufacturers, France-based Zodiac Aerospace, unveiled a set of three seats with one passenger facing forward, one facing backward and another facing forward. The seat bottoms flip up, like the seats at a ballpark, to let passengers board and exit faster.

It's a concept strictly for short-haul flights. The response from airlines at the expo? "Very interested," said Pierre-Antony Vastra, an executive vice president at Zodiac.

The drawbacks: no armrests, and the seat bottom cushions are pretty thin.

Another aircraft interior manufacturer, Thompson Aero Seating of Northern Ireland, was promoting the "Cozy Suite." It's an idea that installs economy seats at an angle, with one slightly behind and to the left of the other, to maximize cabin space. The Cozy Suite also has tilt-up seat bottoms.

But the biggest buzz at the expo was over a small French company called Expliseat, which has developed a seat made of lightweight titanium and composite materials. It weighs about 8.8 pounds, whereas newer economy seats weigh about 24 pounds.

The company promises to save airlines up to $500,000 a year in fuel costs for the average Airbus 320 or Boeing 737 planes. On April 1, the seat won approval by the European Aviation Safety Agency for use on European aircraft.

FLIERS GRIPE YET STILL TAKE SPIRIT

A US-based airline drawing the most complaints per passenger over the past five years is Spirit Airlines. The Florida-based carrier is known for super-tight seating and dozens of fees, including charges for soft drinks and carry-on bags.

But the executives at the ultra-low-cost carrier are probably not sweating the study results, because another report released last week said Spirit also had the highest profit margin of any US carrier in 2013.

"The Unfriendly Skies," a report by the US Public Interest Research Group Education Fund, found that Spirit had the highest complaint rate from 2009 to 2013. It showed that Spirit had 9.44 complaints per 100,000 passengers in 2013, up from 6.75 complaints per 100,000 fliers in 2009.

Spirit's complaint rates were more than three times higher than the second-place airline each year, the report said.

But a report released last week by Airline Weekly gave Spirit reason to smile: Spirit had the highest profit margin (17 percent) of any US airline, compared with 9 percent for Delta and 8 percent for both Southwest and American Airlines.

It's clear that Spirit was not bothered by its complaint rate, because the airline responded to the report with a one-day promotion, boasting that only 0.01 percent of its passengers filed complaints last year.

"That's right, over 99.99 percent of our customers did not file a complaint with the Department of Transportation in 2013," the airline said in its promotion statement.